Marshall JCM800 1992 Super Bass tone stack
Tone stacks and tone controls from amps and pedals, from their schematics, with the stage that drives each one and what it feeds. Turn the knobs and watch the curve. Pin a setting to compare, change any part, and listen through a speaker. Pickups and guitar knobs are on the Guitars page.
The Sweep and Mid controls of the Marshall JCM800 1992 Super Bass, with every part as on the bench.
Swipe sideways for the whole drawing. Tap a part to change it. The source drives the end marked in, and the load hangs on the end marked out.
At this setting
0 dB is the source’s voltage with nothing hooked to it, so the curve shows what the stack cuts. A tube source changes the curve only through its output resistance, not its gain.
Pin this curve keeps what the graph shows, up to three pins in their own colors. Pins stay when you switch stacks, so you can lay a Marshall treble sweep over a Vox one. In a sweep, the lightest line is the knob at 0 and the darkest is 10.
Listen
You hear a picked string through a guitar on full, with 15 feet of cable. Play without it leaves the stack out.
Source and load
Driven by
Driving
The knobs
- Sweep
- Sweep, 1 MΩ linear, a variable resistor (one end free on the drawing). It sets how much resistance sits ahead of the mid cap, so it moves the frequency the mid control works at. Which way it turns isn’t marked, so up is assumed to be less resistance.
- Mid
- Mid, 1 MΩ linear, a variable resistor from the sweep node to the mid cap. Turned down it lets the cap shunt the sweep node to ground, which cuts the mids. Up puts more resistance in the way, for more mids.
Parts
- C1
- 100 nF, Coupling cap off V1a’s plate into the sweep network, .1 µF.
- C2
- 100 pF, The treble path to the volume, 100 pF straight across. Everything below the top end goes through the resistor path through the sweep network.
- R6
- 220 kΩ, From C1 into the Sweep pot, 220 kΩ.
- Sweep
- 1 MΩ, Sweep, 1 MΩ linear, a variable resistor (one end free on the drawing). It sets how much resistance sits ahead of the mid cap, so it moves the frequency the mid control works at. Which way it turns isn’t marked, so up is assumed to be less resistance.
- R7
- 470 kΩ, From the volume’s top to the sweep node, 470 kΩ.
- Mid
- 1 MΩ, Mid, 1 MΩ linear, a variable resistor from the sweep node to the mid cap. Turned down it lets the cap shunt the sweep node to ground, which cuts the mids. Up puts more resistance in the way, for more mids.
- CMID
- 2 nF, The mid cap to ground under the Mid pot. Its value is smudged on the drawing and read as .002 µF.
Source and load
Source: 58 kΩ, the output resistance of the stage before it on the bench at 1 kHz. Load: 1 MΩ, the input of what follows it on the bench, from its impedance at 1 and 10 kHz.
Values, wiring and drawing are taken from the Marshall JCM800 1992 Super Bass on the bench, so the two always match.
Warnings. The summary above the graph turns red if the stack can’t be solved as set, usually from a part at zero or a shorted pot track.
How this tool works. We solve each stack with the circuit bench’s engine: the source stage’s operating point first, then every part at 161 frequencies from 20 Hz to 20 kHz. A stack that’s also in a bench circuit is built from that circuit file, so the two always match.
The curves are small-signal. They don’t show clipping or grid current in the next stage. For the stage before or after a stack, try the gain stage tool.
Spot an error, or want a circuit added? Email info@swellamps.com. A schematic or a clear photo of one helps. Anything you send may be used to improve these models and our desktop app.
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